Optical spectroscopy of crystal-field transitions in the molecular magnet Fe-8

Dipolar transitions in the magnetic cluster Fe-8 and its derivative Fe-8-perchlorate (Fe8PCL) have been studied by coherent-source spectroscopy at frequencies 1.5-33 cm(-1). Four magnetic transitions are observed between 2 and 4 cm(-1) in Fe-8 and Fe8PCL. The temperature behavior of the resonance frequencies, linewidths, and intensities identifies them as transitions inside the spin S = 10 ground state which is split by the crystal field. In both compounds these transitions are characterized by a Gaussian rather than a Lorentzian line shape, which originates from a random distribution of the internal dipolar magnetic fields with a width of approximately 70 mT for Fe-8 and 140 mT for Fe8PCL. We find an excellent agreement of experimental Findings and theoretical description. Several absorption lines are discovered in Fe-8 at frequencies above 15 cm(-1) which could originate from electronic transitions between different multiplets or lattice vibration modes. No corresponding lines are observed in Fe8PCL.